1. Field of the Invention
This invention relates to a dehydrocyclization process for converting C8 isoalkanes and C8 isoalkenes to para-xylene.
2. Prior Art
Para-xylene (PX) is a valuable basic chemicals useful in chemical industry. Commercial xylenes generally comprise three aromatic isomers, inclusive of PX, and may be produced by reforming hydrocarbon feedstocks rich in naphthenes or by dehydrocyclization of naphtha feedstocks, which are rich in C6 to C20 paraffins, or olefins.
For example, U.S. Pat. No. 3,449,461 discloses the production of mixed xylenes and other aromatics by subjecting a paraffinic feed to dehydrocyclization conditions over a sulfided refractory oxide catalyst containing a noble metal such as platinum. In accordance with U.S. Pat. No. 3,766,291, feedstock comprising 3-methylbutene-1 is converted to PX by disproportionation to 2,5-dimethylhexene which is subsequently dehydrocyclized over a catalyst containing at least one Group VIII metal associated with tin in combination with a Group II aluminate spinel support material.
U.S. Pat. No. 3,428,702 discloses the dehydrocyclization of 2,5-dimethylhexene in the presence of H2S using a chromia-alumina catalyst such that 30-40% of the 2,5-dimethylhexene is converted to PX. Other dehydrocyclization processes for converting aliphatic or olefinic hydrocarbons to xylenes are found in U.S. Pat. No. 3,207,801, wherein catalysts based on magnesium oxide, hydoxide or magnesium acid salts are used, and in U.S. Pat. No. 4,910,357 wherein a platinum-loaded, non-acidic, metal modified zeolite support such as ZSM-5 is used.
While these and other methods for the production of xylenes are effective and useful, there is a continuing need in the art to provide a process which is highly selective for producing PX and in high yields from hydrocarbon feedstocks containing less valuable alkane and alkene compounds.
The invention provides a process for producing para-xylene from a feedstock enriched in C8 isoalkane or isoalkene components comprising contacting said feedstock with a dehydrocyclization catalyst under dehydrocyclization conditions of temperature and hydrogen partial pressure, said catalyst comprising a low acidity molecular sieve support having a channel size in the range of about 5-8 angstroms and having a 10 to 12 membered ring structure containing at least two elements selected from the group consisting of Si, Al, P, Ge, Ga and Ti, said molecular sieve further containing at least one Periodic Table Group VIII metal, and recovering a reformate rich in para-xylene.
The process of the invention provides a technique for high conversion of C8 isoalkanes and isoalkenes to xylenes wherein the selectivity ratio of para-xylene to total xylenes present in the reformate is preferably at least about 50 wt %, more preferably at least about 75 wt %.
Hydrocarbon feedstocks, which may be dehydrocyclized in accordance with this invention, include naphtha and paraffinic feedstocks, which are enriched in C8 isoalkanes or isoalkenes. Such feedstocks may generally comprise a mixture of C4 to C20 paraffins and/or alkenes, more preferably C8 to C10 paraffins and/or alkenes. In accordance with this invention, the feedstock is enriched in one or a mixture of C8 isoalkanes or isoalkenes, ie, the feedstock contains greater than 3 wt %, more preferably at least 10 wt %, even more preferably at least 50 wt % and most preferably greater than 90 wt % of said C8 isoalkanes or isoalkenes.
The C8 isoalkane present in the feedstock comprises 2,5-dimethylhexane and the C8 isoalkenes may comprise 2,5-dimethylhexene (2,5-dimethyl-1-hexene, 2,5-dimethyl-2-hexene, 2,5-dimethyl-3-hexene), 2,5-dimethylhexadiene (2,4-, 1,5- and/or 1,3-hexadienes) and 2,5-dimethylhexatriene, including dimers of isobutene.
The dehydrocyclization catalyst used in the present invention comprises a molecular sieve support of low acidity containing at least one Group VIII dehydrogenation metal.
Suitable molecular sieve supports are those having a channel size in the range of about 5 to 8 Angstrons and having a 10 to 12 membered ring structure, such as disclosed in xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d, W. H. Meier, D. H. Olson, C. H. Baerlocher, Elsevier, 4th Edition 1996, the disclosure of which is incorporated herein by reference. These supports contain at least two elements selected from the group consisting of Si, Al, P, Ge, Ga and Ti, most preferably selected from Si, Al and Ti. Exemplary molecular sieves include zeolite L, BEA, ETS-10, ETAS-10, MFI and MTW.
Suitable such supports include the twelve membered ring alkali metal-containing zeolite L aluminosilicates having the general structure:
(0.9xe2x88x921.3)M2/nO:AL203:XSiO2:YH2O
wherein M designates at least one exchangeable alkali metal cation, n designates the valance of M, Y may be any value from 0 to about 9 and x is any value between 5 and 7. Preferably M is potassium. These zeolite L materials and their method of manufacture are more completely described in U.S. Pat. Nos. 4,987,109, 5,849,967 and 5,855,863, the complete disclosures of which patents are incorporated herein by reference.
Other suitable molecular sieve supports include the molecular sieves containing at least one octahedral site and tetrahedral sites of at least one type, such as ETS-10 and ETAS-10. The ETS-10 materials are characterized by the unit empirical formula of
1.0xc2x10.25 M2/nO: TiO2:y SiO2: z H2O
wherein M is at least one cation having a valence of n, y is from 2.5 to 25, and z is from 0 to 100. In a preferred embodiment, M is a mixture of alkali metal cations, particularly sodium and potassium, and y is at least 3.5 and ranges up to about 10.
Titanium silicates of this type are more completely disclosed in U.S. Pat. No. 4,853,202, the complete disclosure of which reference is incorporated herein by reference.
Supports of the ETAS-10 type are generally described by the unit empirical formula of:
(1+x/2)(1.0xc2x10.25 M2/nO): TiO2: x AlO2: y SiO2: z H2O
wherein M is at least one cation having a valence of n, y is from 2 to 100, x is from 0.05 to 5.0 and z is from 0 to 100. In a preferred embodiment, M is a mixture of alkali metal cations, particularly sodium and potassium, and y is at least 2 and ranges up to about 10. Metalloaluminosilicate molecular sieves of this type are more specifically disclosed in U.S. Pat. No. 5,244,650, the complete disclosure of which patent is incorporated hereby by reference.
The molecular sieve support should be of low acidity to minimize isomerization of the para-xylene produced during the dehydrocyclization reaction. Acid sites present in the molecular sieve can be removed by washing the molecular sieve to raise the pH to at least 7, preferably at least 9, as described in U.S. Pat. No. 4,987,109 or by exchanging acid sites on the surface with a cation such as zinc, tin, thallium, lead or alkali or alkaline earth metals. Acid sites can also be blocked by treating the molecular sieve with organosilicon compounds followed by calcination as is known in the art, and by other methods known to those skilled in the art.
In the preferred embodiment, the molecular sieve is sufficiently non-acidic such that it exhibits an alpha value of less than 10, more preferably less than 1, most preferably less than 0.1. The alpha value is a measure of acidity and the test procedure is described in U.S. Pat. No. 3,354,078 as well as in Journal of Catalysis, 4527 (1965), 6278 (1966) and 61,395 (1980), each of which references are incorporated herein by reference.
Because the molecular sieve supports are of micron or submicron size, they are difficult to contain in a fixed bed reactor and would introduce extremely high-pressure drops. The crystals are preferably formed into aggregates such as extrudates, tablets, pills or spherical forms by mixing the crystals with a suitable binder such as alumina, silica or kaolin and water to form a paste, and extruding or otherwise shaping, and cutting the extrudate to form aggregates having a typical dimension of about {fraction (1/32)} to xc2xc inch. Typical binder content may range from about 10-50 wt % of the final aggregate.
Binderless aggregates of Zeolite L of the type disclosed in U.S. Pat. No. 5,849,967 may also be used in the process.
The molecular sieve serves as a support for at least one Group VIII catalytically active metal to form the dehydrocyclization catalyst. The metal can be loaded onto the support by ion-exchange, impregnation or direct synthesis during the manufacture of the molecular sieve. These metals are typically Group VIII metals which include platinum, rhenium and iridium. Other metals can be added to promote the activity and stability of the catalyst. These include tin, iron, germanium and tungsten. Platinum can be introduced by impregnating the crystals either prior to forming the aggregates or the formed aggregate particles with an aqueous solution of a platinum salt or complex such as chloroplatinous acid, hexachloroplatinic acid, dinitrodiaminoplatinum or platinum tetraamine dichloride. Alternatively, platinum can be introduced by ion exchange with ions in the molecular sieve, using a salt such as platinum tetraamine dichloride. Similar compounds can be used to introduce other metals such as rhenium and iridium into the catalyst. Superior catalysts are obtained when at least 90% of the metals added to the catalyst prior to reduction are less than 7 angstrom in size.
The amount of Group VIII metal incorporated in the molecular sieve support can range from about 0.1 to 10wt % of the molecular sieve, more preferably from about 0.5 to 2wt %.
The dehydrocyclization process may be carried out in any suitable fixed bed reactor or other reactor used in reforming processes by passing the C8 enriched feedstream through a bed of the catalyst. For the conversion of naphtha (e.g., C6-C10) and similar mixtures to highly aromatic mixtures, normal and slightly branched chained hydrocarbons, preferably having a boiling range above 40xc2x0 C. and less than about 200xc2x0 C., can be converted to products having a substantial higher octane aromatics content by contacting the hydrocarbon feed with the catalyst at a temperature in the range of 400xc2x0 C. to 600xc2x0 C., preferably 480xc2x0 C. to 550xc2x0 C. at pressure ranging from atomspheric to 40 bar, and liquid hourly space velocities (LHSV) ranging from 0.1 to 15. Hydrogen gas is also introduced, preferably at a ratio of H2/HC of about 1 to 10.
The following examples are illustrative of the invention.